To improve the conductivity of nano-copper ink, a simple chemical process was presented to synthesize oxidation-resistant copper nanoparticles. The copper nanoparticles synthesized by aqueous reduction method were treated by oxalic acid in ethanol. And the particles were then mixed with acrylic resin to prepare conductive ink which was used to fabricate conductive copper patterns on polyimide films by screen printing. TEM, SEM, XRD and XPS were conducted to characterize the synthesized copper nanoparticles and films. After oxalic acid treatment, the copper film showed higher conductivity and its resistivity was as low as 24.1 μΩ·cm after being annealed in the vacuum oven at 250℃ for one hour. With superior oxidative stability and good conductivity, the copper conductive ink treated by oxalic acid is proper for printing electronics.